May 4, 1904
"Acquisition Day"
The Hay-Bunau-Varilla Treaty of 1903 gave to the US, for annual payments, the sole right to control and operate the Panama Canal and a strip of land 5 miles on each side from ocean to ocean. This area was known as The Canal Zone and "Acquisition Day" was May 4, 1904.
http://www.thelatinlibrary.com/imperialism/notes/panamacanal.html